Generally speaking, bottom hemming in the operation of a sewing machine is carried out as follows: as shown in FIG. 5A, the bottom of a fabric 34 is folded over twice and held under a fabric presser 23. A needle 37 drops at a needle drop point 22 and passes through a fabric edge 38 and a folded portion 39 just below the edge 38. The needle is continuously retracted and reinserted to provide blind stitching.
In order to provide overlock stitching, the movement of a sewing thread 40 must be as is shown by the dotted line in FIG. 5B. In this case, the sewing thread 40 should pass through the fabric edge 38 and then through a central line 41 of the folded portion 39 without turning aside to the left or to the right. When the sewing thread 40 passes through the central line 41 of the folded portion 39 without turning aside to the left or to the right, the sewing thread then turns around the fabric edge 38 to obtain overlock stitching so that the sewing thread 40 does not appear on the surface 42 (the lower surface of FIG. 5C) of the fabric 34. This is blind stitching. As seen from the oblique view of the reverse side of the fabric in FIG. 5D, the stitched portion of the sewing thread 40 as represented by dotted lines is visible on side 43 but not on side 42.
When sewing with blind stitches a plain portion of the fabric which has no stepped part on the side seam, it is possible to perform such sewing satisfactorily with the sewing thread 40 invisible on the surface 42 and with no unstitched portion in the fabric. Thus, referring to FIGS. 6A, 6B, 7A and 7B, the location of the fabric guide surface 20 of a hemming guide 5, which is to guide the folded portion 39 for blind stitching in such a way that the folded portion 39 is located exactly at a proper place relative to the needle drop point 22 in contact therewith, varies with the thickness and softness of respective fabrics.
In practice, the fabric concerned is tentatively sewn several times in order to adjust the fabric guide surface 20 very carefully so that the needle 37 can pass through the central point 41 of the folded portion 39. Once the adjustment has been made, the fabric can be sewn with blind stitches very satisfactorily even if it is continuously sewn.
When the sides of a skirt or a body are sewn together lengthwise as is illustrated in FIG. 10, the part sewn is called a stepped part on a side-seam (stepped cross seam). (See FIG. 10). In a fabric containing a stepped part 33 on a side-seam, with a fixed hemming guide so adjusted as to correspond with the fabric, when the stepped part 33 on the side-seam is just before the fixed hemming guide and is not in contact with the fabric guide surface 20, as shown in FIGS. 6A and 6B, the fabric 34 will be guided by the fabric guide surface 20 while being in contact therewith, and so will be sewn quite satisfactorily.
However, when the stepped part 33 on the side-seam has passed through an opening between a fabric contact face 8 of the fixed hemming guide and the edge of a bending guide plate 44 (see FIG. 2), and arrives at the fabric guide surface 20, as shown in FIG. 7A, it is pressed by the fabric guide surface 20, and thereby portions of the fabric just before and behind the stepped part 33 are forced to become hollow portions 35 and 36, as shown by the full line in FIG. 7B.
However, when the stepped part 33 with such hollow portions 35 and 36 advances below the fabric presser 23 and further advance to the needle drop point 22, the needle 37 will drop at a point on a needle point line 45. Thus, the hollow portions 35 and 36 remain unstitched because they do not reach the needle point line 45. This is a great disadvantage in the use of an overlock sewing machine.
In the past, trained sewing operators have each time tried by hand to vary the conditions in which the fabric 34 is guided in order to prevent such a disadvantage in weaving from occurring. But it is a great waste of time and labor to perform such a hand operation in the environment of today's super high-speed sewing. Indeed, this is a main cause of impairment of the efficiency of this kind of sewing operation.